1. The Field of the Invention
The present invention relates to a wide dynamic range transimpedance amplifier. More particularly, the present invention relates to a wide dynamic range transimpedance amplifier with a controlled low cutoff frequency as optical power received at the transimpedance amplifier increases.
2. The Relevant Technology
Fiber optic networks use light signals to transmit data over a network. Although light signals are used to carry data, the light signals are typically converted into electrical signals in order to extract and process the data. The conversion of an optical signal into an electrical signal is often achieved utilizing a fiber optic receiver. A fiber optic receiver converts the optical signal received over the optical fiber into an electrical signal, amplifies the electrical signal, and converts the electrical signal into an electrical digital data stream.
The fiber optic receiver usually includes a photodiode that detects the light signal and converts the light signal into an electrical signal or current. A transimpedance amplifier amplifies the signal from the photodiode into a relatively large amplitude electrical signal. The amplified electrical signal is then converted into a digital data stream.
The optical signals that are converted into electrical signals by the fiber optic receiver, however, can vary significantly in both amplitude and power. The power of the optical signal is often related, for example, to the length (hence loss) of the optical fiber over which the optical signal was transmitted, the laser source power, the efficiency of the photodiode, etc. These and other factors result in optical signals whose incident power at the transimpedance amplifier can vary significantly.
Fiber optic receivers are only able to successfully receive and amplify optical signals that fall within a particular power range. In order for a fiber optic receiver to accommodate a wide range of optical signals, the fiber optic receiver and in particular, the transimpedance amplifier, should be able to detect and amplify very low levels of optical power as well as high levels of optical power. The range of signals that can be successfully amplified is therefore effectively limited by the incident optical power because the fiber optic receiver distorts or clamps signals whose optical power is too large and cannot recognize signals whose optical power is too low.
One problem with current transimpedance amplifiers is that extending the ability of the transimpedance amplifier to amplify signals with more optical power usually diminishes the ability of the transimpedance amplifier to amplify signals with low optical power. In other words, the maximum optical input power that can be accepted by the transimpedance amplifier while meeting signal integrity and bit error rate specifications is usually specified as the input optical overload. The minimum input power is specified as optical sensitivity. The transimpedance amplifier should be designed to maximize the optical overload and minimize the optical sensitivity. In most of the commercial or published transimpedance amplifiers, there is a direct tradeoff between the circuit optical (or current) sensitivity (or equivalent input current noise) and the optical (or current) overload. Some solutions to this problem, such as utilizing clamping circuitry or voltage regulators to assist in the amplification of optical signals with relatively large optical power, add both cost and complexity to the transimpedance amplifier of the fiber optical receiver. Without the aid of additional circuitry, the range of signals that can be successfully amplified is somewhat limited because the circuitry used for automatic gain control and DC cancellation introduces unwanted gain into the transimpedance amplifiers DC cancellation feedback loop at large optical power.
The unwanted gain also has an adverse effect on the low frequency cutoff at higher optical powers. In other words, transimpedance amplifiers do not function at certain frequencies because the low frequency cutoff has been increased. The low frequency cutoff for these types of transimpedance amplifiers is related to the transconductance of the circuitry used for automatic gain control and DC cancellation. Thus, as the current of the input signal increases, the low frequency cutoff of the transimpedance amplifier is adversely affected.
These and other limitations are overcome by the present invention, which relates to a wide range dynamic transimpedance amplifier. In the present invention, the wide dynamic range of the transimpedance amplifier is accomplished in a manner where the gain in optical overload is not completely offset by a loss of optical sensitivity. In addition, the low cutoff frequency does not increase linearly but approaches an upper limit or is controlled as the input current to the transimpedance amplifier increases. This permits, in one embodiment, the transimpedance amplifier to be utilized with legacy systems that may operate at lower frequencies. The low cutoff frequency is controlled as the optical power increases.
In one embodiment, a transimpedance amplifier includes feedback circuitry that provides both automatic gain control, AC attenuation, DC shunting, and a low cutoff frequency at higher optical input powers. A pnp transistor is used in the DC cancellation feedback circuitry such that the emitter impedance of the pnp transistor is controlled, via a feedback loops by the average photodiode current. The emitter is also connected with the input of the transimpedance amplifier.
As the photodiode current increases in response to increased optical power, the emitter impedance of the pnp transistor, which is connected with the input current or signal, decreases. However, unlike a common emitter npn transistor whose collector is connected to the input of the transimpedance amplifier, the pnp transistor does not introduce significant additional gain into the feedback loop as the input signal amplitude increases, thereby keeping the low-cutoff frequency substantially unchanged.
An npn transistor can also be used, for example, when the transimpedance amplifier is connected to the emitter of the npn transistor. Also, the npn is used for in embodiments where a photodiode has its cathode connected to the input of the transimpedance amplifier.
Automatic gain control is achieved because the AC component of the photodiode current is increasingly shunted to ground by the pnp transistor as the average photodiode current increases. The AC component is attenuated at higher amplitudes. As the average photodiode current decreases, the emitter impedance of the pnp transistor decreases and enables low power signals to be passed with little or no attenuation into the main amplifier. This ensures that the optical sensitivity of the transimpedance amplifier is not traded for optical overload. In another example, a shunt feedback transimpedance amplifier also includes feedback circuitry to provide both automatic gain control, AC attenuation, and DC cancellation.
The variable impedance of the feedback circuitry can be achieved using a pnp transistor, an npn transistor, field effect transistors, and the like. In one embodiment, the emitter of an npn transistor is connected with an emitter of a pnp transistor such that current from the photodiode can either be sourced or sunk. Photodiodes that amplify the input current or signal can be accommodated by optimizing, in one example, the pnp transistor to trigger earlier.
In some embodiments of the invention, a transimpedance amplifier includes a first pnp transistor and a second pnp transistor. The emitter terminal of the first pnp transistor is connected to the input of the transimpedance amplifier. The base terminal of the first pnp transistor is connected to the base terminal of the second pnp transistor. The emitter size of the second pnp transistor is some factor N smaller than the emitter size of the first pnp transistor.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.